A radio network will commonly comprise a master radio station and one or more slave radio stations. The slave stations communicate with the master station directly, and may communicate with each other indirectly by means of messages passed via the master station.
In many applications there is a requirement for the slave stations to be powered by battery and therefore there is a requirement for low power consumption in order to maximize the battery life.
In many applications the slave stations, and possibly the master stations, will be mobile, such that the composition of networks is temporary. Therefore there is a requirement for networks to be configured dynamically, with slave stations establishing communication with different masters stations, and vice versa, as the radio stations change location.
The operation of low power master-slave networks is generally as follows. A master station transmits a beacon signal periodically. Messages for slave stations are transmitted by the master station after the beacon signal. Slave stations detect the beacon signal and adopt a power economy scheme synchronized to the beacon signal whereby the slave station activates its receiver when the beacon signal is due to be transmitted, checks whether there is a message to be received, if there is a message to be received the receiver remains active for the duration of the message, and then the slave deactivates its receiver to save power until the next beacon signal is due.
In order to operate in the manner described above, there must be a method by which a slave station can join a new network that it moves within range of by establishing initial communication with the new master station and becoming synchronized to the new network's beacon signal. The method used in prior art systems to establish initial communication and to synchronize is typically as follows.
Referring to FIG. 1, the master station transmits (Tx) a beacon signal B of duration tB at intervals TB. Following each beacon signal is a period when the master station can receive (Rx) signals transmitted by slave stations. A slave station that is not synchronized with the master station samples the radio channel looking for a beacon signal. In order to ensure that the sampling encompasses a beacon signal, the sampling duration tS is a minimum of TB+tB as illustrated in FIG. 1 at C. The sampling interval is TS. In the scenario of FIG. 1, the slave station is out of radio range of the master station and so cannot receive the beacon signal at C. However, the slave station subsequently moves within range of the master station and during the following sampling activity at D detects the beacon signal. Having detected the beacon signal the slave station transmits a signal at E to identify itself to the master station and the master station acknowledges at F. Having detected a beacon signal the slave station activates its receiver synchronously with the beacon signal at G and may receive messages from the master station. When the slave station is sampling the radio channel to detect a beacon signal, its receiver duty cycle is tS/TS=(TB+tB)/TS. As a typical example, TB=15 ms, tB=0.32 ms and TS=1 s, in which case the receiver duty cycle during sampling is 1/65. When the slave station has synchronized with a beacon signal its receiver duty cycle is tB/TB, which for the example is 1/47. If the receiver power consumption is PRx when the receiver is active, the average power consumption is PRx. (TB+tB)/TS during sampling and PRx. tB/TB when synchronized.